The present invention relates to temperature sensitive dry etch processes used in semiconductor fabrication and apparatuses for performing dry etches, and more particularly to controlling the temperature of such processes and such apparatuses to maintain selectivity while providing a high etch rate.
The manufacture of semiconductor devices, and particularly multilayer structures, involves, among other fabrication steps, the patterned etching of areas of the semiconductor surface which are not protected by a pattern of photoresist material. Such etching techniques use either liquid ("wet") etching compositions or gaseous "dry" etching compositions. For example, certain halogen-containing mineral acids such as HCl and HF are known to be active etchants for many of the materials found in semiconductors. When used as wet etchant compositions, these compositions will etch the semiconductor surface isotropically, i.e., in both vertical and lateral directions.
Etching of semiconductor layer materials may also be accomplished using, for example, halogen-containing compounds in the gas phase. Such techniques are broadly categorized as dry etching and include plasma etching, ion beam etching, and reactive ion etching. The use of gas plasma, in the form of gaseous ions, electrons, and neutrals formed from radio frequency discharge, as the etchant provides for substantially anisotropic etching (i.e., etching in the vertical direction only) of the semiconductor surface. Anisotropic etching techniques have become increasingly important as higher density (and thus smaller dimensions) multilayer semiconductors have been introduced.
Such small dimensions and tight tolerances in this industry have created problems which cannot be successfully addressed using photolithography techniques in combination with wet etching. Accordingly, anisotropic etching procedures, in which feature widths must be maintained within certain alignment tolerances, are used. One technique which has been developed along with anisotropic dry etching is the use of etch stop layers.
In an etch stop procedure, an etch stop layer of material is deposited atop an underlying structure. An outer layer, through which desired patterns will be formed, is then deposited over the etch stop layer. The etch stop layer is used to terminate the etching process through the outer layer once that layer has been completely removed in a desired pattern and to protect the underlying structure from the etchant.
Etch stop procedures are designed to have (1) a high outer layer etch rate which (2) etches anisotropically to produce substantially upright sidewalls through the outer layer, and (3) has a high selectivity so that the etchant preferentially etches the material of the outer layer, but not (or only very slowly) the etch stop layer. A preferred etch stop material in the art is silicon nitride because it has properties which are well known and is used widely in semiconductor fabrication. A typical outer layer to be etched comprises silicon dioxide.
One process for obtaining high selectivity in the etching procedure is taught in Blalock et al, U.S. Pat. No. 5,286,344, where a fluorinated etchant containing specific additives is used to form a gas plasma which etches silicon dioxide at a high selectivity with respect to an underlying silicon nitride layer. Other gas plasma etchants with high selectivity are also known.
One problem which has existed in the art for such selective oxide/nitride etches is that if the temperature of the substrate to be etched or the interior of the plasma etching apparatus is too high, the rate of oxide etch drops, and the etch may not be completed. That is, a portion of the insulating oxide layer may remain, preventing good electrical connections from being formed in later fabrication steps. Additionally, if the temperature in the plasma etching apparatus is too high, some of the etchant gas may polymerize and be deposited on the walls of the etched layer. The presence of polymer impedes the rate of etch and results in the formation of angled, not vertical, walls. The polymer may also interfere with completion of the etch through to the etch stop layer. Such overheating problems may occur where the outer layer to be etched is relatively thick, or where the etching process must be continued for a relatively long time period such as in a self-aligned contact etch, or where a high electrode temperature is used.
While some plasma etch apparatuses include a wafer cooling system to maintain the wafer at a constant temperature during the etching process, maintaining a constant wafer temperature does not address the overheating problems in the plasma generating apparatus. Accordingly, the need still exists in the art for a technique for improving the performance of a temperature-sensitive etch process.